4 Ike Calendars and Chronology the astronomical computations and other premisses necessary to determine the precise day in the Egyptian sliding calendar on which this event occurred in any named year cannot be given here. One important record is that the event fell on 16. VIII of the sliding calendar in a year (Year 7 Sesostris III) which from monumental records can be determined as Year 120 of the Twelfth Dynasty. With the postulate assumed and a further minor postulate that the observation of the heliacal rising was made always at Heliopolis before the Ptolemaic period, the year has been deter- mined astronomically as either 1871 * or 1,456 years earlier . The latter alternative is historically impossible. Therefore the commencement of the Twelfth Dynasty is placed in 1990. The record of 1871 is the earliest recorded date of the heliacal rising. Projecting backward, however, the astronomical com- putations show that the heliacal rising occurred on I. I of the sliding calendar in 2769. Further it can be shown that this day was then July 17 (Julian) and that the summer solstice of 2769 occurred (by Heliopolis time) at II p.m. on July 16. Since 1.1 began at sunset on July 16, and the solstice occurred about 6 hours later in each of the next 3 years, it follows that the day of the heliacal rising and the day of the solstice were in effect coincident at this epoch. For this, and other reasons, it is concluded that this curious sidereal year was introduced in 2769, not 1,458 years earlier. Further, there is reason to associate this invention with Imhotep, who was in a late period deified as the Father of Egyptian Science. Imhotep is known to have been the Minister of King Zoser of the Third Dynasty, the i gth King of the line of Menes . at the dawn following the summer solstice, as observed from Heliopolis. In A.D. 138 the event as observed from Alexandria occurred on July 20 Julian, i.e. July 19 Gregorian. 1 The dates are given in the astronomical notation: 1871 must be read as —1871, i.e. 1872 B.C.